The present invention relates to solar collectors. It is particularly useful with respect to flat-plate solar collectors, and is therefore described below in this type of application.
The flat-plate solar collector comprises a metal plate absorber suitably blackened to absorb solar radiation. Within the plate, or attached to it, are tubes or channels that carry the fluid to be heated. This fluid is usually water, but other fluids may be used. The absorber is covered on the side facing the sun by a radiation-transparent window, e.g. of glass or transparent plastic, which reduces cooling by the wind and provides a layer of air acting as insulation between the absorber and the cover window. Multiple glazing is sometimes used to reduce heat losses still further. The rear side of the absorber is normally covered by insulation material of low thermal conductivity to reduce heat losses from that side. A box encloses the absorber and insulation to protect them from the elements and to allow the unit to be transported to the side of use.
It has been proposed to omit the rear insulation material, to replace it by a transparent window, and to provide reflecting surfaces to direct solar and/or sky radiation to the rear side, while the other side is irradiated by direct insolation. In this way more energy is collected from a single absorber. Such a system may be called to double-sided collector.